Embodiments of the present invention relate to an actuator which is operable to reduce the likelihood of “force fighting” when used in conjunction with one or more other actuators as part of an actuator system. Embodiments of the invention are particularly suitable for use in aviation applications, such as in retraction and extension of aircraft landing gear.
Actuators are used in many fields of technology to transform an input signal into motion. In the field of aviation, actuators are used to displace various components into desired orientations/positions, such as the retraction and extension of landing gear, and displacing aerodynamic control surfaces into desired orientations. It has been well established to use hydraulically powered actuator systems for these roles, with a hydraulic fluid providing a medium through which the input signal is transformed into motion. However, hydraulic actuator systems require a complex infrastructure of pipework for containing and transferring hydraulic fluid. Such a complex infrastructure is prone to leakage and spills, thereby reducing the efficiency of operation of the actuator system and representing an environmental hazard. A leakage or spill of hydraulic fluid from the infrastructure pipework may ultimately lead to loss of actuation. The consequences of such a loss of actuation are severe for an aircraft, with the potential to lead to loss of aircraft control due to an inability to adjust the aerodynamic control surfaces and an inability to deploy the landing gear or lock the landing gear in position. In the worst case, failure of the hydraulic actuation systems in an aircraft can result in loss of the aircraft, and consequent loss of life. Unsurprisingly, the maintenance regime which is employed to protect against leakage and catastrophic failure of the infrastructure for the hydraulic actuator systems of an aircraft is necessarily rigorous and expensive, including costs incurred in recharging the hydraulic fluid and disposing of used hydraulic fluid.
To address these concerns, electrically powered actuators have been introduced in which an electric motor provides the source of power for the input signal of an actuator, the input signal then being transformed into motion. Electrically powered actuators avoid the need for the complex infrastructure associated with hydraulic actuation systems and have advantages of improved efficiency and reduced maintenance requirements as compared to hydraulic systems.
It is commonplace in many fields, especially aviation, to have systems requiring the use of two or more actuators which must cooperate together to achieve a desired result. A relevant example is for an aircraft landing gear for use in controlling extension and retraction of the landing gear. For example, U.S. Pat. No. 5,022,609 discloses a landing gear having a mechanism in the form of an over center linkage for locking the landing gear into either an extended (“downlock”) or retracted (“unlock”) position, thereby preventing collapse of the landing gear on landing of the aircraft or undesired deployment of the landing gear while in cruising flight. U.S. Pat. No. 5,022,609 discloses use of a first actuator 130 to unlock this mechanism, and the use of a second larger actuator 116 to subsequently retract the landing gear. Lack of synchronization between cooperating actuators, such as the two actuators of U.S. Pat. No. 5,022,609, can result in the actuators acting in opposition to one another. This undesired phenomenon is known as “force fighting”. Known hydraulic actuator systems as conventionally used in landing gear systems mitigate some of the effects of force fighting through the cooperating actuators being operated from a common valve. The use of a common valve and some compressibility in the hydraulic fluid used in the cooperating actuators helps to minimize the effects of any force fighting between the cooperating actuators. However, it has been found that electrical actuator systems are vulnerable to unnecessary stresses being imposed on the cooperating actuators as a result of force fighting. This vulnerability is caused by various factors, including increased actuator stiffness due to the use of electrical actuators, without there being alleviation of force fighting stresses by compressibility of hydraulic fluid. Left unchecked, these stresses would result in failure of one or more of the cooperating actuators. A solution would be to employ complex control circuitry to ensure precise synchronized operation between cooperating actuators and thereby avoid the occurrence of force fighting. However, such control circuitry would add to the weight and size of the actuator systems, as well as adding complexity to the system design. Alternatively, designing actuators for an electrical actuator system which are able to withstand the additional stresses imposed as a result of “force fighting” would also result in increased actuator weight and size, which would thereby offset the benefits of using electrically powered actuators over known hydraulic actuator systems. Therefore, there is a need for an improved actuator which reduces or eliminates the undesired stresses caused by “force fighting” while minimizing the weight and size of the actuator.